Conventional photomultiplier tubes include a vacuum envelope containing a photocathode, several dynodes and an electron collector. Light entering the tube through a window and incident on the photocathode causes electrons to be emitted by the photocathode. The electrons impinge on the successive dynodes, causing electron multiplication by secondary emission. After impingement on the last dynode, the electrons are collected and delivered on an output lead of the tube to provide an output signal which is representative of the input light.
A hybrid photomultiplier tube includes a photocathode, electron focusing electrodes and an electron bombarded photodiode anode. Electrons emitted by the photocathode are focused onto the photodiode. The electrons penetrate into the photodiode material and create electrode-hole pairs, causing a multiplication effect. Gain is produced by the photodiode rather than the dynodes, as in the conventional photomultiplier tube.
A hybrid photomultiplier is disclosed by L. K. van Geest et al in "Hybrid Phototube With Si Target", SPIE, Vol. 1449, Electron Image Tubes and Image Intensifiers, II, 1991, pages 121-134. A photomultiplier tube using both dynodes and an impact ionization diode for electron multiplication is disclosed in U.S. Pat. No. 3,885,178 issued May 20, 1975 to Goehner.
A bias voltage on the order of 10 kilovolts is typically applied between the anode and cathode of a hybrid photomultiplier tube. The electrons are accelerated by the applied field and bombard the photodiode anode, which results in multiplication gain. However, depending on the photodiode material, 20-30% of the electrons backscatter off the diode surface at varying angles. Some of the electrons strike the inside surface of the tube wall, causing charging of the tube wall. The charges modify the potential inside the tube, causing a defocusing of the electron beam and operational instability. In addition, for a sufficiently large acceleration voltage, x-rays may be generated at the diode surface due to electron deceleration. The x-rays can strike the inside surface of the tube wall, resulting in ionization or positive charging of the surface. Again, the net result is electron defocusing and operational instability. A curved channel electron multiplier, wherein part of the outer surface was coated with a silver film to determine the effect of distributed capacitance on various parameters, is disclosed by K. C. Schmidt et al in "Continuous Channel Electron Multiplier Operated in the Pulse Saturated Mode", IEEE Trans. Nucl. Sci., June 1966, p. 100-111.
One prior art approach to the wall charging problem in photomultiplier tubes involves a partially conductive coating, such as green or black chrome oxide, on the inside surface of the tube wall. However, the coating must be highly resistive to prevent short circuiting of the tube electrodes. Thus, the coating is not particularly effective in reducing the effect of wall charging. Furthermore, the tube life may be reduced by outgassing from suck coatings into the vacuum envelope.